1. Field of the Invention
The present invention relates to a solenoid suitable for use as an electromagnetic spill valve or the like for a fuel injection system.
2. Description of the Related Art
Recently, an electromagnetic spill valve, controlled by a computer, for timed spilling of high-pressure fuel has been proposed. The spilling operation of this known electromagnetic spill valve is controlled by an electronic method instead of a mechanical method. Referring to FIG. 13 showing an electromagnetic spill valve which closes a spill passage when the same is energized, flow passages 33 and 36 are formed in the lower portion of a valve body 31 so as to open respectively in the side surface and the bottom surface of the valve body 31 such that and the flow passages 33 and 36 communicate with each other through of a spring chamber 32. The flow passages 33 and 36 and the spring chamber 32 serve as portions of the spill passage. A needle valve 27 is supported for axial sliding movement in the upper portion of the valve body 31 with its points facing a seat surface 40 formed in the upper end of the spring chamber 32. The needle valve 27 is biased upward so as to be separated from the seat surface 40 by a compression coil spring 28 placed in the spring chamber 32. An electromagnet 25 is mounted on the valve body 31 so as to surround the upper portion of the valve body 31. The electromagnet 25 comprises a magnetic core 25a and an electric solenoid 25b. A plate-like armature 26 is fixed to the upper end of the needle valve 27. When the electric solenoid 25b is not energized, an air gap G is formed between the lower surface of the armature 26 and the upper end of the magnetic core 25a. The value of the gap G is dependent on the lift of the needle valve 27. The value of the gap G is, for example, 0.1 mm when the needle valve 27 is seated on the seat surface 40 to close the spill passage. When the electric solenoid 25b of the electromagnet 25 is magnetized, the armature 26 is moved downward by magnetic attraction to move the needle valve 27 downward against the resilience of the compression coil spring 28. When the needle valve 27 is seated on the seat surface 40, the flow passages 33 and 36 are disconnected from each other.
An engine for a vehicle, for example, requires a fuel injection system capable of injecting high-pressure fuel at a high rate so that of an exhaust gas discharged contains less harmful gases. The consequently, the lift of the needle valve of an electromagnetic spill valve of such a construction must be at least 0.3 mm to provide an opening sufficiently large. However, the value of the air gap G of the electromagnetic spill valve increases when the lift of the needle valve is increased. Since the magnetic attraction of the electromagnet is substantially inversely proportional to the square of the value of the air gap, the maximum lift of the needle valve must be limited to about 0.2 mm, which is contradictory to preferred conditions.